Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Other imaging processes use electrostatic or ionographic printing. Electrostatic printing is printing where a dielectric receptor or substrate is “written” upon imagewise by a charged stylus, leaving a latent electrostatic image on the surface of the dielectric receptor. This dielectric receptor is not photosensitive and is generally not reuseable. Once the image pattern has been “written” onto the dielectric receptor in the form of an electrostatic charge pattern of positive or negative polarity, oppositely charged toner particles are applied to the dielectric receptor in order to develop the latent image. An exemplary electrostatic imaging process is described in U.S. Pat. No. 5,176,974.
In contrast, electrophotographic imaging processes typically involve the use of a reusable, radiation sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which may also be referred to as “light” or actinic radiation, may include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using an electrically-biased development roller to bring the charged toner particles into close proximity to the photoreceptive element. The polarity of the development roller should be the same as that of the toner particles and the electrostatic bias potential on the development roller should be higher than the potential of the imagewise discharged surface of the photoreceptor so that the toner particles migrate to the photoreceptor and selectively develop the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The transfer of an image typically occurs by one of the following two methods: elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”).
Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by balancing the relative surface energies between the ink, a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, force, and toner rheology. An exemplary elastomeric assist/adhesive image transfer process is described in U.S. Pat. No. 5,916,718.
Electrostatic assist or electrostatic transfer refers generally to a process in which transfer of an image is primarily affected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner. Electrostatic transfer may be influenced by surface energy, temperature, and force, but the primary driving forces causing the toner image to be transferred to the final substrate are electrostatic forces. An exemplary electrostatic transfer process is described in U.S. Pat. No. 4,420,244.
In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high force with or without heat. In the cleaning step, any residual toner remaining on the photoreceptor after the transfer step is removed. Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to radiation of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Electrophotographic imaging processes may also be distinguished as being either multi-color or monochrome printing processes. Multi-color printing processes are commonly used for printing graphic art or photographic images, while monochrome printing is used primarily for printing text. Some multi-color electrophotographic printing processes use a multi-pass process to apply multiple colors as needed on the photoreceptor to create the composite image that will be transferred to the final image receptor, either by via an intermediate transfer member or directly. One example of such a process is described in U.S. Pat. No. 5,432,591.
In one exemplary electrophotographic, multi-color, multi-pass printing process, the photoreceptor takes the form of a relatively large diameter drum to permit an arrangement of two or more multi-color development units around the circumference perimeter of the photoreceptor. Alternatively, toners of varying colors can be contained in development units that are arranged on a moveable sled such that they can be individually moved into place adjacent to the photoreceptor as needed to develop a latent electrophotographic image. A single rotation of the photoreceptor drum generally corresponds to the development of a single color; four drum rotations and four sled movements are therefore required to develop a four-color (e.g. full color) image. The multi-color image is generally built up on the photoreceptor in an overlaid configuration, and then the full color image is transferred with each color remaining in imagewise registration, to a final image receptor, either directly or via an intermediate transfer element.
In an exemplary electrophotographic, four-color, four-pass full color printing process, the steps of photoreceptor charging, exposure, and development are generally performed with each revolution of the photoreceptor drum, while the steps of transfer, fusing, cleaning, and erasure are generally performed once every four revolutions of the photoreceptor. However, multi-color, multi-pass imaging processes are known in which each color plane is transferred from the photoreceptor to an intermediate transfer element on each revolution of the photoreceptor. In these processes, the transfer, cleaning and erasure steps are generally performed upon each revolution of the photoreceptor, and the full-color image is built up on the intermediate transfer element and subsequently transferred from the intermediate transfer element to the final image receptor and fused.
Alternatively, electrophotographic imaging processes may be purely monochromatic. In these systems, there is typically only one pass per page because there is no need to overlay colors on the photoreceptor. Monochromatic processes may, however, include multiple passes where necessary to achieve higher image density or a drier image on the final image receptor, for example.
A single-pass electrophotographic process for developing multiple color images is also known and may be referred to as a tandem process. A tandem color imaging process is discussed, for example, in U.S. Pat. No. 5,916,718 and U.S. Pat. No. 5,420,676. In a tandem process, the photoreceptor accepts color toners from development units that are spaced from each other in such a way that only a single pass of the photoreceptor results in application of all of the desired colors thereon.
In an exemplary four-color tandem process, each color may be applied sequentially to a photoreceptive element that travels past each development unit, overlaying each successive color plane on the photoreceptor to form the complete four-color image, and subsequently transferring the four-color image in registration to a final image receptor. For this exemplary process, the steps of photoreceptor charging, exposure, and development are generally performed four times, once for each successive color, while the steps of transfer, fusing, cleaning, and erasure are generally performed only once. After developing the four-color image on the photoreceptor, the image may be transferred directly to the final image receptor or alternatively, to an intermediate transfer member and then to a final image receptor.
In another type of multi-color tandem imaging apparatus, each individual color's development unit may include a small photoreceptor on which each color's contribution to the total image is plated. As an intermediate transfer member passes each photoreceptor, the image is transferred to the intermediate transfer member. The multi-color image is thereby assembled on the intermediate transfer element in overlaid registration of each individual colored toner layer, and subsequently transferred to the final image receptor.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images, and are therefore preferred for high resolution, multi-color printing applications.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like. Exemplary liquid toners and liquid electrophotographic imaging process are described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227-252.
The liquid toner composition can vary greatly with the type of transfer used because liquid toner particles used in adhesive transfer imaging processes must be “film-formed” and have adhesive properties after development on the photoreceptor, while liquid toners used in electrostatic transfer imaging processes must remain as distinct charged particles after development on the photoreceptor.
Toner particles useful in adhesive transfer processes generally have effective glass transition temperatures below approximately 30° C. and volume mean particle diameter between 0.1-1 micron. In addition, for liquid toners used in adhesive transfer imaging processes, the carrier liquid generally has a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. This is particularly true for cases in which multiple colors are sequentially deposited and overlaid to form a single image, because in adhesive transfer systems, the transfer is promoted by a drier toned image that has high cohesive strength (commonly referred to as being “film formed”). Generally, the toned imaged should be dried to higher than approximately 68-74 volume percent solids in order to be “film-formed” sufficiently to exhibit good adhesive transfer. U.S. Pat. No. 6,255,363 describes the formulation of liquid electrophotographic toners suitable for use in imaging processes using adhesive transfer.
In contrast, toner particles useful in electrostatic transfer processes generally have effective glass transition temperatures above approximately 40° C. and volume mean particle diameter between 3-10 microns. For liquid toners used in electrostatic transfer imaging processes, the toned image is preferably no more than approximately 30% w/w solids for good transfer. A rapidly evaporating carrier liquid is therefore not preferred for imaging processes using electrostatic transfer. U.S. Pat. No. 4,413,048 describes the formulation of one type of liquid electrophotographic toner suitable for use in imaging processes using electrostatic transfer.
Photoreceptors generally have a photoconductive layer that transports charge (by an electron transfer or hole transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support, such as a conductive drum or an insulative substrate that is vapor coated with aluminum or another conductor. The surface of the photoreceptor can be either negatively or positively charged so that when activating electromagnetic radiation imagewise strikes the surface of the photoconductive layer, charge is conducted through the photoreceptor to neutralize, dissipate or reduce the surface potential in those activated regions to produce a latent image.
An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and thereby extend the service life of the photoconductive layer. Other layers, such as adhesive layers, priming layers, or charge injection blocking layers, are also used in some photoreceptors. These layers may either be incorporated into the photoreceptor material chemical formulation, or may be applied to the photoreceptor substrate prior to the application of the photoreceptive layer or may be applied over the top of photoreceptive layer. A “permanently bonded” durable release layer may also be used on the surface of the photoreceptor to facilitate transfer of the image from the photoreceptor to either the final substrate, such as paper, or to an intermediate transfer element, particularly when an adhesive transfer process is used. U.S. Pat. No. 5,733,698 describes an exemplary durable release layer suitable for use in imaging processes using adhesive transfer.
Many electrophotographic imaging processes make use of intermediate transfer members (ITM's) to assist in transferring the developed toner image to the final image receptor. In particular, in a multipass electrophotographic process, these ITM's may contact the final image formed on the photoreceptor to assist transfer of entire image to the ITM. The image may then be transferred from the ITM to the final image receptor, typically through contact between the ITM and the final receptor.
In a tandem process, individual photoreceptors layer the images formed by the component colors on the ITM. When the entire image is composed in this manner it is typically transferred to the final image receptor. However, U.S. Pat. No. 5,432,591, for example, discloses the use of an offset roller to remove the entire image from a photoreceptor and transfer it to the final image receptor in a multi-pass liquid electrophotographic process. In various embodiments, the ITM may be an endless belt, a roller or a drum.
One continuing problem in electrophotography is to ensure that the toner particles transfer efficiently from the photoreceptor to the final image receptor, either directly or indirectly using an intermediate transfer member. Frequently, a noticeable percentage of the toner layer is left behind at each transfer step, resulting in reduced image fidelity, low optical density and poor image quality on the final image receptor, and toner residues on various machine surfaces that must be efficiently cleaned. This problem of low transfer efficiency is particularly troublesome for liquid electrophotographic toners, wherein slight variations in the carrier liquid content of the toned image can control the efficiency of elastomeric transfer or electrostatic transfer of the image from the photoreceptor or to a final image receptor.
Various attempts have been made to use transfer layers to assist transfer of liquid toned images from a temporary image receptor (e.g. a photoreceptor) or to a final image receptor (e.g. paper). For electrostatic or ionographic printing processes, a dielectric peel layer has been used to augment transfer from a temporary image receptor (see e.g. U.S. Pat. No. 5,176,974). Alternatively, an adhesive-coated protective laminating film has been used to augment transfer of liquid toners from a temporary electrographic receptor (see e.g. U.S. Pat. No. 5,370,960).
For liquid electrophotographic printing, a substantially continuous and uniform coating of a high viscosity or non-Newtonian liquid transfer layer has been applied to assist toner particle transfer from a photoreceptor and to a final image receptor using elastomeric or adhesive transfer. A variety of peelable or releasable transfer assist films have also been used in liquid electrophotographic printing processes wherein the photoreceptor has a surface release characteristic and elastomeric (adhesive) transfer is used to transfer the toned image from the photoreceptor surface. The peelable or releasable film is said to improve toner transferability, provide high quality and high fidelity multicolor images irrespective of the type of final image receptor or image receiving material, and improve storage stability of the final images (see e.g. U.S. Pat. No. 5,648,190, U.S. Pat. No. 5,582,941, U.S. Pat. No. 5,689,785 and U.S. Pat. No. 6,045,956).
Each of these methods for using a transfer assist material in a liquid electrophotographic printing process is directed to multi-pass processes that use adhesive or elastomeric transfer of the image from a specially-prepared photoreceptor having a surface release character, either directly to a final image receptor or indirectly to an intermediate transfer element and then to the final image receptor. Each of these methods involves the application of the transfer assist material as a substantially uniform or continuous film on the photoreceptor. Because the transfer assist material is deposited not only where the toned image is developed, but also in non-imaged background areas, a portion of the transfer material ends up in the background regions on the final image receptor, adding to the expense of using the transfer assist material and potentially degrading the appearance of the final image on plain paper. The art continually searches for improved liquid toner transfer processes, and for methods and materials that allow liquid toner particles to transfer more completely, producing high quality, durable multicolor images on a final image receptor at low cost.